[gdb/tdep] Fix larl handling in s390_displaced_step_fixup

[gdb/tdep] Fix larl handling in s390_displaced_step_fixup

On s390x-linux with target board unix/-m31, I run into:
...
(gdb) PASS: gdb.guile/scm-lazy-string.exp: bad length
print ptr^M
$1 = 0x804006b0 <error: Cannot access memory at address 0x804006b0>^M
(gdb) FAIL: gdb.guile/scm-lazy-string.exp: ptr: print ptr
...

A minimal example is:
...
$ gdb -q -batch -ex "set trace-commands on" -x gdb.in
+file scm-lazy-string
+break main
Breakpoint 1 at 0x4005d2: file scm-lazy-string.c, line 23.
+run

Breakpoint 1, main () at scm-lazy-string.c:23
23        const char *ptr = "pointer";
+step
24        const char array[] = "array";
+print ptr
$1 = 0x804006b0 <error: Cannot access memory at address 0x804006b0>
...

If we delete the breakpoint after running to it, we have instead the expected:
...
+delete
+step
24        const char array[] = "array";
+print ptr
$1 = 0x4006b0 "pointer"
...

The problem is in displaced stepping, forced by the presence of the breakpoint,
when stepping over this insn:
...
  0x4005d2 <main+10>      larl    %r1,0x4006b0
...

With normal stepping we have:
...
(gdb) p /x $r1
$2 = 0x3ff004006b0
...
but with displaced stepping we have instead (note the 0x80000000 difference):
...
(gdb) p /x $r1
$1 = 0x3ff804006b0
(gdb)
...

The difference comes from this code in s390_displaced_step_fixup:
...
  /* Handle LOAD ADDRESS RELATIVE LONG.  */
  else if (is_ril (insn, op1_larl, op2_larl, &r1, &i2))
    {
      /* Update PC.  */
      regcache_write_pc (regs, from + insnlen);
      /* Recompute output address in R1.  */
      regcache_cooked_write_unsigned (regs, S390_R0_REGNUM + r1,
                                      amode | (from + i2 * 2));
    }
...
where the "amode |" adds the 0x80000000.

Fix this by removing the "amode |".

Tested on s390-linux, with native and target board unix/-m31.

Approved-By: Ulrich Weigand <uweigand@de.ibm.com>